The Effect of High Humidity on the Dynamic Mechanical Properties and Thermal Transitions of an Epoxy-polyamide Adhesive

Exposure of a cured epoxy-polyamide adhesive to high humidity resulted in a substantial decrease in the complex dynamic tensile modulus of the material. The effect could be reversed and the original modulus essentially regained by drying the adhesive. Thermal transitions in the dry adhesive were displaced by approximately 40°C to lower temperatures in the wet material; this effect could be reversed by drying.

Strength losses experienced by aluminium joints bonded with this adhesive on exposure to humid conditions could not be regained by removal of water from the joints. The mode of failure of these joints changed progressively from wholly cohesive to predominantly adhesive on exposure to high humidity.

It is concluded that the primary role of water in joint degradation is to displace adhesive from its metal substrate and not to induce cohesive failure of the adhesive.